The present invention relates to the control and monitoring of an electric generator set including an engine and an alternator particular, the present invention relates to the sensing by a genset controller of one or more currents flowing within the alternator.
Electric generator sets (or xe2x80x9cgensetsxe2x80x9d) are widely used to provide electric power. A genset typically includes an engine coupled to an alternator, which converts the rotational energy from the engine into electrical energy. The terminal voltage of a genset is proportional to both the magnetic flux density within the alternator, and the speed of the engine. The magnetic flux density is typically determined by controlling an armature voltage or field current on the alternator, while the speed of the engine is typically determined by an engine governor.
It is known to control the operation of a genset with a genset controller. A new invention in the controlling of gensets relating to a thermal protection subroutine is described in a related patent application filed on the same date herewith, entitled xe2x80x9cMETHOD AND APPARATUS FOR PREVENTING EXCESSIVE HEAT GENERATION IN AN ALTERNATOR OF A GENERATOR SETxe2x80x9d, which is hereby incorporated herein by reference. In accordance with this invention, the currents flowing within an alternator of a genset are monitored in order to determine whether the currents are excessive, such that excessive heat exposure and damage to the alternator could result, and the genset controller causes the currents within the alternator to be reduced if they are becoming too great. This is in contrast to many conventional systems that lack such a feature as part of the genset controller, and in which the genset itself is required to have a circuit breaker to prevent excessive currents in the alternator.
In order for a genset controller to provide effective control of the currents of an alternator to preclude excessive heat exposure, the genset controller must be capable of measuring the currents within the alternator. In particular, the genset controller must be able to measure the currents both when they are within or close to the rated current level, as well as when they are significantly higher than the rated current level, or even an order of magnitude (i.e., 10 times) greater than the rated current level. This is not possible for conventional genset controllers, since such controllers are only capable of measuring currents within a given range, e.g., 0 to 150% of the rated current.
Further, although the genset controller must be able to measure currents that are both below the rated current level and also very high above the rated current level, the genset controller must still be able to obtain measurements of the currents that are accurate, particularly when those currents are at or near the rated current level. Without such accurate measurements, the genset controller lacks a reliable basis for determining whether the currents within the alternator are truly excessive, and for determining what are the appropriate control signals to provide feedback to the genset. Yet, if a conventional genset controller was tailored to be able to measure currents of up to an order of magnitude greater than the rated current level of an alternator, the conventional genset controller would then lack sufficient accuracy in measuring low-level currents at or near the rated current level.
It would therefore be advantageous if a new method and apparatus were developed for sensing the currents within an alternator even when those currents varied over a wide range. It would particularly be advantageous if the method and apparatus allowed for the sensing of currents that were both less than the rated current level and also currents that were substantially higher than the rated current, e.g., within an order of magnitude of the rated current. It would further be advantageous if the method and apparatus maintained highly accurate sensing capabilities for currents at or near the rated current level even though the method and apparatus allowed sensing of the currents that were much higher than the rated current level. It would additionally be advantageous if the method and apparatus could be implemented for sensing such a wide range of currents with a minimum of circuit elements and/or software, and was relatively inexpensive and simple to implement on a genset controller.
The present inventors have discovered that it is possible to provide in a genset controller a multiplicative gain switching device within a voltage signal conditioning unit, which both allows for the sensing of voltages representative of alternator currents at or near the rated current level when switched to a first position, and also allows for the sensing of voltages representative of alternator currents up to an order of magnitude (10 times) greater than the rated current level when switched to a second position. Through the use of this multiplicative gain switching device and certain additional limited software programming, a wide range of current values can be sensed accurately, especially at the lower current levels, without requiring significant numbers of additional or specialized circuit elements.
In one embodiment of the invention, the multiplicative gain switching device operates so that an adjustment resistor is coupled in parallel with an existing feedback resistor within a differential amplifier when alternator currents are increasing above a first threshold near the rated current level. This decreases the gain of the differential amplifier. Further, the multiplicative gain switching device also operates so that the adjustment resistor is decoupled from the feedback resistor when alternator currents are decreasing below a second threshold also near the rated current level. This increases the gain of the differential amplifier. Thus, by switching on and off the switching device and thereby switching the adjustment resistor on and off, the gain of the differential amplifier is varied to account for the changes in the alternator currents.
When the adjustment resistor is coupled in parallel to the existing feedback resistor, the genset controller is configured to switch in its operation to a high current algorithm, which takes into account the low amplification of the differential amplifier. Similarly, when the adjustment resistor is no longer in parallel with the existing feedback resistor, the genset controller is configured to operate using a low current algorithm to take into account the high amplification of the differential amplifier. Consequently, the genset controller is able to sense low current levels at or near the rated current level with great accuracy due in part to the high gain on the differential amplifier, and is also able to sense high current levels up to an order of magnitude greater than the rated current level using effectively the same circuitry by reducing the gain on the differential amplifier.
In particular, the present invention relates to a system for sensing alternator current levels. The system includes the combination including an operational amplifier having an input and an output, an input resistor connecting the input to a signal indicative of an alternator current level, and a feedback resistor connected between the input and the output. The system further includes an adjustment resistor and a switching element coupled in series between the input and the output, in parallel with the feedback resistor, and a processor coupled to the output. The processor is operable, based upon a current indication related to a level of alternator current indicated at the output, to control the operation of the switching element such that the switching element is closed when the current indication increases to exceed a first threshold, and such that the switching element is opened when the current indication decreases to fall below a second threshold.
The present invention further relates to a system for accurately sensing current levels within an alternator. The system includes an amplification means for amplifying or reducing a first signal indicative of a current level within the alternator to produce a second signal indicative of the current level, and a modification means for adjusting the level of amplification or reduction of the amplification means. The system additionally includes a processing means for controlling the modification means and for processing at least one of the second signal and a third signal based upon the second signal to determine a current measurement value.
The present invention additionally relates to a method of accurately sensing current levels within an alternator. The method includes providing a differential amplifier configured to receive a first indication of a current level within the alternator at a first input port and to provide a second indication of the current level at an output port. The method further includes providing a processor in communication with a switching element of the differential amplifier and the output port, receiving the first indication of the current level, and determining at the processor at least two measured current values based upon the second indication. The method additionally includes switching a status of the switching element to reduce a gain of the differential amplifier when the measured current values increase from being below a first threshold to exceed the first threshold, and switching the status of the switching element to increase the gain of the differential amplifier when the measured current values fall from above a second threshold to below the second threshold.